NOPE - this is one phone call. She was on the phone when Trayvon noticed that Z was following him, so he put his hoodie on, When he walked fast (or Ran) when Trayvon asked Z "Why are you following me" and when Z said "What are you doing around here". She heard this all. Listen to the link you can hear her yourself. Don't take my word for it.
Respectfully, you are incorrect. Here is the timeline:
18 : 54 : --
Martin calls girlfriend while walking back to father's home from a 7-Eleven. According to his girlfriend, a man had been following him. She stated that Martin had slowed down in an attempt to identify [Zimmerman]. She told Martin to just run home. Martin declined and stated that he would "walk fast".
19 : 09 : 34
Zimmerman calls 911 and reports a suspicious black man "who looks like he's up to no good or he's on drugs or something..." with his hand in his waistband.
19 : 11 : 59
Zimmerman tells dispatch that [Martin] is running towards the back entrance. This is also the point where the alleged epithet is uttered. Dispatch asks Zimmerman if he is following [Martin] and instructs him not to continue. For approximately 1 1/2 minutes, Zimmerman follows Martin.
19 : 12 : --
Martin receives a call from his girlfriend, a call which lasted around four minutes. Martin indicated that he'd evaded [Zimmerman], however the man had apparently caught back up to Martin. On this call the girlfriend stated that she overheard Martin demanding to know why [Zimmerman] was following him. [Zimmerman] responded by demanding to know what Martin what doing there. His girlfriend also stated she heard Martin being pushed down, because his headset dislodged from his ear and the call is disconnected.
19 : 16 : --
Call with Martin's girlfriend is disconnected.
19 : 17 : 11
Martin is shot
There were two calls between Martin and the girlfriend. He called her. They spoke about him being followed and he told her that he would 'walk fast'. Later, she calls him back and overhears the 'why are you following me?'/'what are you doing here?' questions.
Regarding your back turned question, Zimmerman wouldn't have had to ask the question with his back turned. Imagine him walking back to his vehicle. From behind, Martin asks 'Why are you following me?'. As Zimmerman turns around, he asks 'What are you doing here?'. Martin punches him in the nose. The act of throwning the punch dislodges his earpiece. A fight ensues that, per the timeline, goes on for one minute, ending in Martin's death.